Do not take an adverb that ends in “-ly” and make it a...

[yo4561]

In the All the Verbal book, the following is said:
Do not take an adverb that ends in “-ly” and make it a comparison by changing the ending to “-er”. Instead, add “more”.
Incorrect example: Charles runs QUICKER than James.
Correct example: Charles runs MORE QUICKLY than James.

Does this depend on the verb though?
If I swap out "runs" with "is", then you would need to say "Charles is quicker than James", as "Charles is more quickly than James" would be incorrect.

Thank you

[esledge]

I think verbs of being might be the only exception here, because your example "Charles is quicker than James" actually uses quicker as an adjective. It's common to see "is/are/were/will be (adjective)" structures in English.

Correct: Charles is louder than James. (Loud/louder is describing the people, not how either one exists, per se.)
Correct: Charles sings more loudly than James. (Loudly describes how Charles "sings.")

[yo4561]

I think verbs of being might be the only exception here, because your example "Charles is quicker than James" actually uses quicker as an adjective. It's common to see "is/are/were/will be (adjective)" structures in English.

Correct: Charles is louder than James. (Loud/louder is describing the people, not how either one exists, per se.)
Correct: Charles sings more loudly than James. (Loudly describes how Charles "sings.")

Hi Emily, thank you for your response!
To clarify:
-The "more quickly" is an adverbial phrase because it describes how James runs?
-You would use the is/are/were/will be as verbs with "-er" words only for descriptors (e.g. to describe someone)?

To my ear, "Charles sings louder than James" sounds okay too. Will the GMAT really pick for this issue?

Thank you again

[esledge]

-The "more quickly" is an adverbial phrase because it describes how James runs?

Yes. The candle in direct sunlight melted more quickly than the candle inside. "More quickly" describes how it "melted"; it could not describe sunlight or either of the candles.

-You would use the is/are/were/will be as verbs with "-er" words only for descriptors (e.g. to describe someone)?

Yes, I think, though there might be some exceptions that I'm overlooking or that are not really the type of -er words you mean (e.g. words that just happen to end in -er, such as "ever," which is an adverb). The -er words I think you care about are comparative adjectives (generally) made by putting an -er ending on a regular adjective. Those will typically go with is/are/were/will be verbs.

To my ear, "Charles sings louder than James" sounds okay too. Will the GMAT really pick for this issue?

Not so much! And if it did, your process of finding splits would show that you have a "louder than" vs. "more loudly than" decision, and that should prompt you to ask "what is loud?" If the answer is a verb, play it safe with "loudly."